Magnetic amplifier circuits, especially for arc-welding equipments



Nov. 22, 1966 R. MALATIER ETAL 3,287,625

MAGNETIC AMPLIFIER CIRCUITS, ESPECIALLY FOR ARC-WELDING EQUIPMENTS FiledSept. 24. 1963 2 Sheets-Sheet l INVENTORS ROBERT MALAT/ER RlfiHARDTHEURET MAW/4 ATTORNEYS Nov. 22, 1966 R. MALATIER ETAL 3,237,625MAGNETIC AMPLIFIER CIRCUITS, ESPECIALLY FOR ARC-WELDING EQUIPMENTS FiledSept. 24, 1963 2 Sheets-Sheet 2 mvswroras ROBERT MAL-A TIER RICHARDTHEUKET 'ATTORN EYS United States Patent 3,287,625 MAGNETIC AMPLIFIERCIRCUITS, ESPECIALLY FOR ARC-WELDING EQUIPMENTS Robert Malatier andRichard Thenret, both of 33 Place du 1er Mai, Dijon, France Filed Sept.24, 1963, Ser. No. 311,127 Claims priority, application France, Sept.24, 1962, 910,258; Sept. 17, 1963, 947,776 6 Claims. (Cl. 32389) Thepresent invention relates to magnetic amplifier circuits and inparticular but not exclusively to those which are employed inarc-welding equipments.

It is well known that a magnetic amplifier comprises a load winding, acontrol win-ding and a polarizing winding. In certain applications suchas arc welding, it should be possible to vary the intensity of theoutput current of the magnetic amplifiers supplied from the rectifiedoutput of a transformer. This intensity can be varied by modifying thenumber of ampere-turns of the control winding. In the known systems, arheostat is employed for that purpose. However, this involves a certainnumber of disadvantages.

It is in fact necessary to add an auxiliary rectifier which is capableof delivering a high current and which is therefore relatively costly.The regulating rheostat is of delicate mechanical design and has afairly considerable overall size; more especially, it develops a hightemperature as a result of theJoule effect in the resistance wire.

In addition, the welding current is subjected to the voltage variationsof the supply network, since the voltage drop is reflected back incascade by the presence of the magnetic amplifier.

Finally, it is desirable to provide vigorous cooling for the whole ofthe rectifiers and the rheostat, which involves the utilization of arelatively powerful fan.

Apart from the fairly considerable risk of breakdown, mention may alsobe made of the ageing of the components, and more particularly of therheostat, large overall size together with considerable weight, and thenecessity for periodical maintenance.

The present invention makes it possible especially to overcome thesedrawbacks.

To this end, according to the invention, the control ampere-turns of themagnetic amplifiers are varied by supplying their control windings witha rectified current of variable intensity obtained from a grid-type.rectifier controlled by a phaseshifting network.

The controlled-grid rectifier is preferably a silicon thyratron.

The phase-shift network provides a horizontal phase displacement alongthe time axis which enables the output current of the controlled gridrectifier to be varied from zero up to almost the maximum value.

According to a first form of embodiment, the network which effects thephase displacement comprises a winding having a central tapping in whichthe current is in phase with the anode current of the rectifier, thisWinding forming part of a group comprising a phase-shifting capacity, asupplementary rheostat, and a transformer supplying the grid of thethyratron. The phase-shift angle is determined by a set of resistances,with which is associated a second set of resistances which shunt therheostat and permit the range of regulation to be corrected independence on the selected intensity.

According to a particular feature of the invention, the transformerwhich serves to supply the grid of the thyratron supplies a voltagegreater than the striking voltage of the thyratron, so as to takeaccount of the voltage variations of the supply network. This too highvoltage supplied by the transformer is peak-clipped by a suitablecircuit, comprising a resistance and a Zener diode, so as to give avotlage with a square-top wave-form corresponding to the average voltagerequired for striking the thyratron. In this way, there is obtained awell-defined point of intersection, and the variation of the voltage ofthe supply system has no longer practically any effect on the strikingangle of the thyratron.

A system of this kind has numerous advantages, especially when it isutilized in an arc-welding equipment. In fact it enables a very steadywelding current to be maintained in the case of variations of voltage ofthe supply system, since the number of ampere-turns of the magneticamplifier practically does not vary.

In addition, it is presented in the form of an extremely robust shieldedelectronic unit which does not include any component necessitatingperiodical maintenance and developing no appreciable heat. Inconsequence, it is not necessary to-provide any cooling system whichrepresents an economy in electric power, the risks of breakdown arealmost non-existent, there is no maintenance, the overall size is verysmall and the weight is negligible. The unit does not comprise any fan,is silent and is extremely simple and easy to handle.

According to a second form of embodiment of the invention, thephase-shift network only comprises a fixed capacity and a resistancevariable bet-ween very wide limits, in the form of a rheostat mounted onthe electrode holder within easy reach of the operator. This arrangementpermits a continuou regulation of the phase of the voltage applied tothe transformer which supplies the thyratrons of the control circuit ofthe self-saturating inductances, that is to say it definitely permitscontinuous regulation of the welding current by the operator during theactual operation of welding.

In this second form of embodiment, there is provided a fixedpolarization of these inductances, and also a safety circuit havingessentially the function of protecting the operator in the event ofaccidental contact with the two poles of the apparatus and the rectifierdiodes together ample and without limitation, in its application to theconstruction of a direct-current genenator for electric arc welding.

FIG. 1 is a basic diagram of a first form of embodiment of theinvention.

FIG. 2 is a basic diagram of a second form of embodiment of theinvention.

Finally, FIGS. 3a and 311 show diagrammatically an electrode holderdesigned for use with this second form of embodiment.

In the diagram of FIG. 1, there is shown at T a main three-phasetransformer, for example of 10 k-va., which does not have a fallingcharacteristic. The secondary windings of this transformer supplyrectifier systems constituted by the silicon rectifiers D D D to theterminals of which are connected condensers C C C serving as aprotection against reverse voltage surges.

One the output of each rectifier is connected at self-saturableinductance constituted by a load winding and a control-polarizationwinding.

The self-satura'ble inductances have been designated by S S S the loadwindings by f f f and the controlpolarizati-on windings by e e e Thecontrol-polarization winding '(e e e is traversed by a half-waverectified current, making it possible to obtain a number of polarizationor control ampereturns which adjust the minimum or maximum value of theoutput current.

. According to the invention, the control winding (e e a is traversed bya half-wave rectified current having an intensity variable in accordancewith the value of the control angle of the grid of a thyratron Thyacting as a controlled grid rectifier. There is thus obtained avariation of the number of control ampere-turns which enables the outputof the rectifier unit to be varied. To this end, the control windings ofthe magnetic amplifiers are connected in a network supplied from theoutput of the thyratron Thy.

The thyratron is supplied in turn by an auxiliary transformer T havingan output of two watts for example. The alternating voltage of thistransformer is substantially greater than that which is necessary at thecontrol electrode of the thyratron for the excitation of all theelements; for example, it is about six times greater than this minimumvoltage. A voltage of this amplitude cannot be applied directly to thegrid of the thyratron, since it exceeds the maximum value of thedirect-current voltage applicable between the control electrode and thecathode, and which is for example volts.

It is quite clear that a lower alternating voltage, for example of 3volts, which has not been peak-clipped, causes the striking of thethyratron. However, it has been found in this case that a variation ofvoltage of the supply system results in a variation of the strikingangle as a result of variation of the point of intersection. This toohigh voltage is therefore peak-clipped by means of a system comprisingfor example a resistance R and a Zener diode Z. There is thus obtained avoltage of square-top wave-form, slightly trapezoidal, which correspondsto the average voltage required for striking the thyratron. By thismeans, a well-defined point of intersection is obtained and thevariation of the voltage of the supply system causes practically novariation in the striking angle of the thyratron.

The output of the thyratron is controlled by a horizontal phasedisplacement along the time axis. This phase displacement is obtained bya phase-shift unit comprising a winding g with a central tapping iconnected in the circuit of the transformer T Between the winding g andthe transformer T is connected a phase-shift condenser C The windingwith a central tapping in which the current is in phase with the anodecurrent of the thyratron forms part of a group which comprises, inaddition to the capacity C, and the transformer T a set of resistancesand a supplementary rheostat Rh.

The phase-shift angle is determined by the position of a contact memberon one of the resistances R to R or R to R and at the same time on oneof the resistances R to R or R to R which shunt the rheostat Rh. Theselatter resistances R to R are intended to correct the range ofregulation as a function of the intensity selected. As indicated above,the output of the transformer T is connected on the one hand to thecathode of the thyratron and on the other to the peak-clipping assemblyconstituted bythe Zener diode mounted in shunt across the terminals ofthe thyratron and by the resistance R The diagram is completed by areversing switch I which enables the control winding of the inductancesof the magnetic amplifiers to be reversed and the positioning of theintensity to be communicated by variation of the phaseshift angle bymeans of the resistances R to R the fine adjustment being obtained bythe balancing resistances R to R3.

The circuit is of course completed .by the usual protection systems.Thus, there is provided a condenser C for filtering thecontrol-polarization current, a condenser C for protecting the windings,a condenser C connected to the output terminals for raising the strikingvoltage, a leakage resistance R for the grid of the thyratron, and adischarge resistance R 0 for the condenser C The device according to theinvention thus eliminates the control rheostat, which is a costly andfragile apparatus, necessarily amply dimensioned in order to dissipatethe Joule effect produced in the resistance wire. The silicon thyratronand the phase-shift system have no mechanical parts to require anymaintenance, do not give rise to any Joule heating and are absolutelyinvariable with time.

The utilization of a silicon thyratron for varying the controlampere-turns of the inductances enables a flexible and reliable assemblyto be produced, since the thyratron has properties of strength, hermeticsealing, invariability and good performance under temperature, and itslife is unlimited if the rules of use provided by the manufacturers areobserved.

In the diagram of FIG. 2, the transformer T comprises a three-phaseprimary and :a six-phase secondary which, by means of two groups ofrectifiers D to D and D' to D' Each group rectifying the same halfwave,supplies two separate channels K and K connected through shuntingresistance RSH. Channel K supplies welding voltage and channel Ksupplies striking voltage.- The inductances S' to 8' comprise, betweenthe main current winding Ch to Ch a control winding e to e and a fixedpolarizing winding P to P which are separate and independent. Thewindings P to P are supplied through the resistance R by two diodes Dand D mounted in push-pull and by one of the windings of a single-phaseauxiliary transformer T having three secondary windings. In addition,this transformer supplies on the one hand the control circuit of themain circuit through the intermediary of the phase-shift network C-D-Rh,the transformer T .and the thyratrons Thy; and Thy on the other hand, itsupplies a safety circuit which comprises an electro-rnagnetic contactorCE, a condenser C a transformer TP which supplies, through theintermediary of the diodes D and D the fixed polarization voltage,limited by the Zener diode Z and which, through the intermediary of thediode D and the resistance R supplies the grid voltage of the thyratronsThy and Thy,

The operation of this system is as follows:

The protection of the operator is based on the fact that, byconstruction, the voltage at no-load on the electrode- .earth terminalsT+ and T is 6 volts. For putting the generator into operation, it isnecessary for the shunting resistance (R in the diagram of FIG. 2) to beless than 10 ohms. On no-load, with the contactor CE open, no weldingvoltage is available on the electrode. If the welder happens to touchsimultaneously the twowelding terminals he only receives a current lessthan 0.005 ampere, which means that he is completely protected. Thiscurrent will be supplied by the winding F in series with thepolarization voltage transformer T1? and the capacity C the resistanceof the operators body (greater than 1,000 ohms) does not permit ofobtaining a secondary voltage sutficient to cause the release of thethyratrons- Thya and Thy At the moment when the operator begins tostrike the are, he causes 'a dead short-circuit between the weldingoutputs; in consequence, the transformer TP is supplied by analternating current since it is then in series with the condenser C andthis combination is coupled in parallel to the terminals of the windingP The diodes D and D connected as push-pull rectifiers, rectify the twohalfwaves of the voltage supplied by the secondary of the transformerTP; there will therefore appear a direct-current voltage limited to thepeak value of the Zener diode Z this voltage is applied to therespective grids of the thyratrons Thy and Thy through the resistances RR and R and causes the release of the thyratrons Thy and Thy Therectified direct current passes through the excitation coil of theelectromagnetic contactor CE and causes the closure of the weldingcircuit. The are is established and its very low resistance does notpermit the de-excitation of the thyratrons Thy and Thy This will takeplace when the arc is broken. At that moment, the transformer TP is nolonger traversed by an alternating current but only by the peak ripplesof the rectified current which is definitely insufficient to overcomethe effect of the polarization applied to the grids of Thy and Thy Thispolarization is obtained by a potential divider R -R connected to theterminals of the polarization current rectifiers, the polarizationvoltage being stabilized by the Zener diode Z and the resistance R whichprevents any accident to the thyratrons as a result of an accidentalshortcircuit or of a break in the resistance R This polarization iscontinuously applied to the grids of the thyratrons through theresistances R R and R A time constant obtained by means of one of thecondenscrs C to C and its resistances R to R enables the generator tocontinue to maintain its voltage for one, two or three seconds after thewelder has broken the arc, this having the purpose of preventing theoperation of the contactor CE at each interruption of the arc. The diodeD permits the condensers C to C to be rapidly charged in the case wherethe arc is broken close to the beginning of the striking.

It is essential to observe that since the arc is always interrupted as aresult of the electrode being moved away, the contactor CE always breaksthe circuit while the latter is open, which prevents any deteriorationof its contacts.

FIG. 3a completes the diagram of FIG. 2 in the sense that it representsdiagrammatically the rheostat Rh and the line Q which connects it to thepoints X and Y of the control circuit comprising the phase-shiftcondenser CD, while FIG. 3b represents the practical circuit of thisrheostat. The rheostat is mounted in a protection casing and is fixed onthe insulating sleeve Q of the cable V which leads the welding currentto the moving electrode EL, by means of a clamping collar M. Itcomprises a knurled knob N, the rotation of which enables the resistanceto be varied from 0.1 to SOKQ for example, starting from a referencepoint P corresponding to a well-defined and known value.

What we claim is:

1. In an arc welding system supplying a direct current output throughload terminals to form a welding arc; an alternative current source; afirst transformer having a three-phase primary, secondary windings andrectifying device thereon to supply a rectified output; a magneticamplifier supplying said direct current welding output from saidrectified output, said magnetic amplifier having load windings, controlwindings and polarization windings; the improvement comprising acircuit, comprising, in combination, at least one grid-controlled solidstate thyratron with an anode therein supplying said control windings ofsaid magnetic amplifier, and a phase shifting network having a windingwith a central tapping in which the current is in phase with said anodeof said solid state thyratron, said winding of said phase shiftingnetwork forming part of a group comprising an auxiliary transformer andat least one variable resistor for varying the phase angle of thecurrent supplying said grid of said solid state thyratron through saidauxiliary transformer to provide a rectified control current to saidcontrol windings of said magnetic amplifier, means for electricallyconnecting said winding and central tapping with said auxiliarytransformer and means for electrically connecting the other end of saidwinding through said at least one variable resistor to said auxiliarytransformer whereby the number of ampere-turns of said control windingsmay be modified to vary the intensity of said direct current weldingoutput.

2. In an arc welding system supplying a direct current output throughload terminals to form a welding arc; and alternate current source; afirst transformer having a three-phase primary, secondary windings andrectifying device thereon to supply a rectified output; a magneticamplifier supplying said direct current welding output from saidrectified output, said magnetic amplifier having load windings, controlwindings and polarization windings; the improvement comprising acircuit, comprising, in combination, at least one grid-controlled solidstate thyratron with an anode therein supplying said control windings ofsaid magnetic amplifier, and a phaseshifting network having a windingwith a central tapping in which the current is in phase with said anodeof said solid state thyratron, said winding of said phase-shiftingnetwork forming part of a group comprising an auxiliary transformer, asupplementary rheostat, a first set of resistors and a second set ofresistors for varying the phase angle of the current supplied by saidphase-shifting network, by adjusting said first set of resistors, saidsecond set of resistors shunting said supplementary rheostat forcorrecting the range of adjustment, said current supplying said grid ofsaid solid state thyratron through said auxiliary transformer to providea rectified control current to said control windings of said magneticamplifier, means for electrically connecting said winding and centraltapping with said auxiliary transformer and means for electricallyconnecting the other end of said winding through said supplementaryrheostat and said first and second resistors to said auxiliarytransformer, whereby the number of ampere-turns of said control windingsmay be modified to vary the intensity of said direct current Weldingoutput.

3. A circuit as claimed in claim 2 including a peakclipping devicecomprising a resistance and a Zener diode electrically connected fromsaid auxiliary transformer to the terminals of said solid statethyratron whereby any excess voltage, provided by said auxiliarytransformer, greater than the striking voltage of said solid-statethyratron is limited.

4. A circuit as claimed in claim 1 further comprising a second group ofrectifying devices connected in pushpull and means for electricallyconnecting said rectifying devices to one of said secondary windings ofsaid first transformer and to said polarization windings of saidmagnetic amplifier whereby said rectifying devices supply saidpolarization windings With .a fixed polarization.

5. A circuit as claimed in claim 1 in which a striking voltage issuperimposed on the welding voltage.

6. A circuit as claimed in claim 1 further including a safety networkcomprising an electromagnetic contactor in series with said loadwindings of said magnetic amplifier, a safety transformer having primaryand secondary windings, a first circuit connected to the load terminalscomprising, in series, said secondary winding of said first transformersupplying said magnetic amplifier said primary windings of said safetytransformer and a capacitor; a second circuit comprising said secondarywindings of said safety transformer, a rectifier, at least one storagecapacitor, at least one solid-state thyratron, a fixed polarizationcircuit for said thyratron and a control winding for controlling saidcontactor, whereby said secondary winding of said safety transformersupplies said rectifier to provide said thyratron through said storagecapacitor with a polarization voltage opposed to said fixed polarizationvoltage; and whereby said thyratron supplies said control winding ofsaid contactor only for small load impedance, said storage capacitorkeeping said thyraof said welding arc.

8 Lobosco et a1. 219-131 Miller 323-89 Greene 321-25 Brown 321-25 XDavis 323-60 JOHN F. COUCH, Primary Examiner. RICHARD M. WOOD, LLOYDMCCOLLUM,

7 tron in conductive state for a short time after extinction 3,041,4453,092,769 3,123,761 References Cited by the Examiner 3,128,422 UNITEDSTATES PATENTS 5 3,128,440

3/1928 Churchward 219-135 8/1947 Hall 219-132 X 11/1953 Welch 219-13210/1957 Huge 321-25 X 1/1962 Rebuffoni 219 131 10 W 3/1962 Bergmann219-131 Examiners.

. E. RAY, Assistant Examiner.

1. IN AN ARC WELDING SYSTEM SUPPLYING A DIRECT CURRENT OUTPUT THROUGHLOAD TERMINALS TO FORM A WELDING ARC; AN ALTERNATIVE CURRENT SOURCE; AFIRST TRANSFORMER HAVING A THREE-PHASE PRIMARY, SECONDARY WINDINGS ANDRECTIFYING DEVICE THEREON TO SUPPLY A RECTIFIED OUTPUT; A MAGNETICAMPLIFIER SUPPLYING SAID DIRECT CURRENT WELDING OUTPUT FROM SAIDRECTIFIED OUTPUT, SAID MAGNETIC AMPLIFIER HAVING LOAD WINDINGS, CONTROLWINDINGS AND POLARIZATION WINDINGS; THE IMPROVEMENT COMPRISING ACIRCUIT, COMPRISING, IN COMBINATION, AT LEAST ONE GRID-CONTROLLED SOLIDSTATE THYRATRON WITH AN ANODE THEREIN SUPPLYING SAID CONTROL WINDINGS OFSAID MAGNETIC AMPLIFIER, AND A PHASE SHIFTING NETWORK HAVING A WINDINGWITH A CENTRAL TAPING IN WHICH THE CURRENT IS IN PHASE WITH SAID ANODEOF SAID SOLID STATE THYRATRON, SAID WINDING OF SAID PHASE SHIFTINGNETWORK FORMING PART OF A GROUP COMPRISING AN AUXILIARY TRANSFORMER ANDAT LEAST ONE VARIABLE RESISTOR FOR VARYING THE PHASE ANGLE OF THECURRENT SUPPLYING SAID GRID OF SAID SOLID STATE THYRATRON THROUGHAUXILIARY TRANSFORMER TO PROVIDE A RECTIFIED CONTROL CURRENT TO SAIDCONTROL WINDINGS OF SAID MAG-